The present invention generally relates to improved telecommunications systems and in particular to an improved digital/POTS telecommunications system. Still more particularly, the present invention relates to an improved digital/POTS splitter design.
The basic functions and requirements for POTS Splitter are well defined in TIE1.4/98-007R5, Annex E, which is hereby incorporated by reference. The POTS splitter is used to split “Plain Old Telephone System” (POTS) voiceband signals from Asymmetric Digital Subscriber Line (ADSL) signals traveling over the same telephone line.
In conventional systems, the POTS splitter is designed as a LC low-pass filter. With regard to ADSL signals, a low-pass filter provides protection from the high-frequency transients and impedance effects that occur during POTS operation, e.g., ringing transients, ring trip transients, and off-hook transients and impedance changes. With regard to POTS voice band service, the low-pass filter provides protection from ADSL signals which may impact through non-linear or other effects remote devices, e.g., handset, fax, voice band modem, etc., and central office operation.
TIE1.4/98-007R5, Annex E specifies acceptable ranges for insertion loss in the voice band, return loss in the voice band, and attenuation distortion in the ADSL band, among other requirements. These requirements make the common POTS splitter design, which incorporates a differential pair of conventional LC low-pass filter circuits, less than ideal for this purpose. Because the inductor used in a conventional LC low-pass filter circuit is frequency independent in the voice range, it is very hard to meet each of the requirements above at same time. It would therefore be desirable to provide an improved low-pass filter circuit for a POTS splitter which optimizes the TIE1.4 requirements.